1. Field of the Invention
This invention relates to a anti-skid control apparatus for a vehicle braking system which can prevent the locking of the wheels.
2. Description of the Prior Art
Such an anti-skid control apparatus for a vehicle braking system is known that includes a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, said fluid pressure control valve device receiving control signals of a control unit measuring the skid condition of the wheel to control the brake fluid pressure to said wheel cylinder; a hydraulic reservoir which, when the brake fluid pressure to said wheel cylinder is decreased with control of said fluid pressure control valve device, reserves the brake fluid discharged through said fluid pressure control valve device from said wheel cylinder; a pressure fluid supply conduit connecting said master cylinder with said fluid pressure control valve device; and a fluid pump for returning the brake fluid from said hydraulic reservoir into said pressure fluid supply conduit.
When the fluid pressure control valve device is provided for each of four wheels, and the fluid pressure of them are independently controlled, there is no problem on control operation. Or when the fluid pressure control valve device is provided for each of front wheels, and for both of rear wheels in common, there is no problem on control operation. In the latter case, the one common fluid pressure control valve device is controlled on the basis of the lower one of the speeds of the rear wheels.
However, in the above cases, there or four fluid pressure control valve devices are used. Accordingly, the whole anti-skid control apparatus is large-sized, and very heavy. Since the fluid pressure control valve device is expensive, it requires high cost.
For example, it is considered that the brake fluid pressures of the front wheels are controlled by the two fluid pressure control valve devices respectively in the diagonal or X-type conduit system, and the brake fluid pressures of the rear wheels are controlled in common with the front wheels. However, when the vehicle runs on the road, the right and left sides of which are considerably different in frictional coefficient from each other, there is the fear that the one rear wheel being diagonal to the one front wheel on the higher frictional coefficient side is locked. In that case, the steering of the vehicle becomes unstable, and that is very dangerous.
Further, it is considered that proportioning valves are provided for the rear wheels, respectively. However, the brake fluid pressures of the rear wheels increases in proportion to the input fluid preSssures to the proportioning valves. The fear of locking cannot be avoided.